The welding of laminated packing material by means of induction heat is used in the packaging industry. One example of such a laminated material is a carrier layer of paper and cardboard, aluminium foil and thermoplastic coatings along the inner and outer surfaces of the laminate.
The technique of induction welding is based on the fact that the magnetic field around a conductor, generated by an alternating current is capable of inducing a current in an adjoining electrically conducting material, which depending on the resistance of the material, brings about a more or less pronounced warming up of the material. Thus in induction welding an inductor loop or coil of the same configuration as that which the desired weld is to receive is fitted adjoining the laminate containing the aluminium foil, whereupon the laminate is pressed together with the material to which it is to be joined. The aluminium foil is heated in a pattern corresponding to the coil by means of appropriately chosen current, frequency and processing period. The material can be heated to a temperature which is high enough to seal adjoining layers of thermoplastic, causing the combined thermoplastic layers to fuse together, thus producing a tight and durable seal.
A modern method for manufacturing liquid packages includes a plane plastic-coated web which is formed to a tube in a filling machine. The tube is filled with contents and sealed off along separate, narrow zones located at right angles to the longitudinal direction of the tube. It is customary in this context to use induction welding for the transverse welds which divide the packing material tube 2 into individual packages.
In the manufacture of a continuous pipe or tube from a plane packing material web, edge zones of the web are overlapped. The overlapping edge zones are combined by heating and surface fusion.
A customary method for forming continuous pipes or tubes from a plane web consists in carrying out the welding together of the overlapped edge zones, the so-called longitudinal joint weld, with the help of hot air. The combined thermoplastic layers are heated to sealing temperature, and pressed together while being cooled at the same time. Often, one edge zone is provided with a narrow strip of thermoplastic which is welded first to one edge zone of the material web before the two edge zones are brought together for the purpose of forming a continuous tube. A joint is obtained towards the inside of the package which is wholly covered by thermoplastic. Otherwise, one edge of the joint is open towards the interior of the package, which means that the carrier layer of paper or cardboard of the packing laminate makes direct contact with the contents. The contents can penetrate into the carrier layer and dissolve the paper fibres. Likewise, the welding of the thermoplastic strip is generally carried out with hot air in a manner similar to that of the longitudinal joint.
This method, however, involves a relatively long warming up period for the filling machine, in order to heat the hot-air element. Furthermore, it is necessary at short production stops in the filling machine to reheat the joint in order to obtain a continuous longitudinal joint without interruption. Such an arrangement requires a great number of movable parts in order to make the process fully automatic. For example, the pressure roller presses together the joint has to be turned away and a separate heating element has to heat the area underneath the pressure roller.